Hi,
"Cart Variant" was for VM2.x, VM3.x doesn't have "Cart Variant"
To create "Cart Variant" in VM3.x, choose "String" as custom field type, but don't forget to choose
"Cart Attribute -> YES"
"Cart Input -> YES"
then the rest of the procedure is the same as demonstrated in the video tutorials. You can also use "Image" type custom field as well. Image swapping works with Image Custom Field as well.
Regards.